Nine soils with distince properties and Se levels were selected to test a fractionation procedure of soil Se based on sequential extraction.Soil Se was fractionated into readily available Se(fraction I,extracted by 0.5 M NaHCO3),slowly available Se(fraction II,extracted by 0.1 M NaOH-0.1 M Na4P2O7),amorphous oxide-occluded Se(fraction Ⅲ,extracted by acid ammonium oxalate)free oxide-occluded Se (fraction VI,extracted by dithionite-citrate-bicarbonate buffer solution)and residual Se(fraction V,determined by NHO3-HClO4 digestion of the final soil residue).The recovery of soil Se(the sum of all fractions over total soil Se determined independently)by this procedure was from 88.1% to 110.9%,mean 99.2%±6.4% for the test soils.The sum of fractions I and II,provided a good measure of available Se in soils and the percentage of fraction I plus II over the total soil Se,tentatively defined as Se availability index.could be used to indicate soil Se status and predict Se deficiency.
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